Fast neutron nuclear reactors which are cooled with liquid sodium comprise a core consisting of the fuel assemblies carried on a horizontal support which is arranged inside a vessel containing liquid sodium, which constitutes the primary fluid of the reactor, into which the whole set of fuel assemblies is inserted. The liquid sodium which comes into contact with these assemblies, i.e., the primary sodium, is not used directly for transmitting the heat from the reactor core to the steam generator, which produces the steam for supplying the turbine.
The primary sodium which circulates inside the vessel, and is caused to do so by pumps, provides for heating of the secondary liquid sodium inside intermediate heat exchangers.
The pumps used for circulating the primary sodium and the intermediate heat exchangers are arranged inside the reactor vessel, and are consequently inserted, in the same way as is the core of the reactor, into the primary sodium which fills the vessel.
The secondary liquid sodium circulates in a closed circuit between the intermediate heat exchangers and the steam generators where the heat which is carried by this secondary liquid sodium provides for vaporization of the feed water and the production of the steam which is used by the turbine.
The reactor vessel is closed at its upper end by a horizontal slab of great thickness which has openings formed in it allowing the passage of the components of the reactor and the measuring devices associated with the latter which are generally of elongated shape and are arranged vertically. The lower parts of these components or measuring devices pass inside the vessel through openings which are provided in the slab, while the upper parts of such components or measuring devices rest on the slab outside the vessel, which assures that such components or measuring devices are maintained in position. A sealing device which is provided between the component and the slab allows the component to close off the openings in the slab in a sealed manner. In particular, the primary pumps and the intermediate heat exchangers have such arrangements provided at the openings in the slab.
On the other hand, a large dimensional opening is provided at the central part of the slab vertically above the reactors core. This opening is designed to receive a unit which allows the reactor to be controlled by displacing rods which absorb neutrons in the core, and a device for handling the fuel assemblies so as to carry out the loading and unloading stages of the reactor core.
The control unit and the handling device are carried on a horizontal platform which can be rotated about a vertical axis referred to as the "large rotating stopper", and which closes off the opening provided in the slab.
The control unit and the device for handling the fuel assemblies are mounted on a circular platform which is referred to as the "small rotating stopper", and the dimensions of this are smaller than those of the large rotating stopper, it being rotatably mounted on the large rotating stopper about a vertical axis which is different from the axis of rotation of the latter.
The large rotating stopper has a diameter which is greater than the diameter of a cylinder arranged on a vertical axis which is of such a size that it would contain the whole of the core.
The components, which must remain in a fixed position when movement of the large rotating stopper arranged at the central portion of the slab occurs, consequently must be arranged on the slab in the region of the latter which is located at the periphery of the large rotating stopper.
This, for example, applies to the primary pumps and the intermediate heat exchangers which must be arranged all around the large rotating stopper and in abutment with the peripheral region of the slab.
This arrangement of the intermediate heat exchangers, which are eight in number in the case of fast neutron nuclear reactors of a power of 1200 MW which are at present being built, makes it necessry to provide a slab and a vessel of a large diameter in order to be able to arrange components which are of appreciable weight on the slab at the peripheral region of the latter.
The vessel of the nuclear reactor is consequently of appreciably larger diameter than the diameter of the core, generally of the order of three times the diameter of the core.
The vessel which holds the liquid sodium in which the reactor core and the main components of the reactor are immersed is, furthermore, surrounded by a safety vessel which further increases the space occupied by the complete vessel assembly. In order to reduce constructional costs of the nuclear reactor, and in order to be able to reduce the amount of primary liquid sodium which fills this vessel, it is desirable to be able to reduce the diameter of the vessel.